<!DOCTYPE html>
<html>

<head>
    <meta charset="utf-8">
    <title>Vue 测试实例 - 菜鸟教程(runoob.com)</title>
    <script src="https://cdn.staticfile.org/vue/2.4.2/vue.min.js"></script>
</head>

<body>
    <div id="app">
        <p style="font-size:25px;">计数器: {{ counter }}</p>
        <button @click="counter++" style="font-size:25px;">点我</button>
    </div>
    <script type="text/javascript">
        var vm = new Vue({
            el: '#app',
            data: {
                counter: 1
            }
        });
        vm.$watch('counter', function(nval, oval) {
            alert('计数器值的变化 :' + oval + ' 变为 ' + nval + '!');
        });
        console.log(vm.$watch);
    </script>
</body>

</html>